Move mixed content check to PresentationRequest ctor.

[mfoltzgoogle]

Addresses Issue #362.

[mfoltzgoogle]

@mounirlamouri PTAL.

Note I could also move the sandboxing flag check to the ctor as well (removing it from the steps for start(), reconnect(), and getAvailability()). WDYT?

[tidoust]

That looks good to me.

About the sandboxing flag, cannot sandboxing conditions change over time? For instance, the sandbox attribute of an iframe element may be removed at any time, which resets the sandboxing flag set:

When an iframe element’s sandbox attribute is removed while it has a nested browsing context, the user agent must empty the iframe element’s nested browsing context’s iframe sandboxing flag set as the output.
http://www.w3.org/TR/html51/semantics-embedded-content.html#element-attrdef-iframe-sandbox

Now, one could argue that once you have an PresentationRequest instance, you should be able to use it, even if sandboxing conditions change (and I don't see any good reason for an app to change the value of the sandbox attribute on the fly).

The check could also be done in the constructor and in start(), reconnect(), etc.

[mounirlamouri]

lgtm, I think you should open an issue for the sandboxing flag. FWIW, a quick read of the spec let me think the sandboxing flags are set at navigation/creation but a quick read of the Blink code let me think it's updated dynamically.